The present invention relates to direction finder systems, and more particularly to a direction finder system using spread spectrum techniques.
Cellular telephone system operators, as an example, spend a lot of time looking for sources of interference. A typical approach is to use a sensitive receiver/spectrum analyzer coupled with a directional antenna. Guided by either a magnitude display or an audible tone amplitude, the directional antenna is moved to maximize the received signal from the interfering source, thus determining the signal""s direction of arrival. This method has the advantage of simplicity, using readily available hardware. However its disadvantage is the limited accuracy of the direction measurement due to the width of the main pattern lobe of the directional antenna.
Another well known approach is to use two or more antennas and compare the relative phase or time of arrival of the interfering signal at each antenna. By knowing the relative phases or times of arrival and how the array is constructed, the direction of arrival of the interfering signal as it sweeps across the antennas may be inferred, as is described in xe2x80x9cElectronic Measurementsxe2x80x9d 2d. Ed. By McGraw-Hill Book Company (1952) by Terman and Pettit at pages 465-467. In some systems the two or more antennas are mounted in a fixed relationship with each other and are physically turned to equalize the times of arrival or phases of the signal being measured, thus pointing the way to the interference source. In other systems the antennas are fixed.
The problem with a multiple antenna approach is that some form of receiver or down converter is required for each antenna/channel. Each receiver""s characteristics need to be matched to the others to achieve accuracy. It may require very heroic adjustment to multiple receivers"" frequency response and gain characteristics to track.
In the three decade period around World War II a scheme was published that used one receiver and its input was electronically switched from antenna to antenna to allow the use of one receiver for all antennas, eliminating the receiver matching problem. The single receiver was in effect multiplexed between the antennas. The receiver""s output was then demultiplexed and used to drive a cathode ray tube (CRT) display to create an instantaneous direction indication system. If a signal with a coherent carrier, such as amplitude modulation, is measured, it is feasible through the use of averaging or phase lock techniques to make antenna-to-antenna carrier phase comparisons to determine the time of arrival of the signal. However, with modern digitally modulated signals, especially spread spectrum signals, the instantaneous phase of the RF carrier is hidden by the modulation. Simultaneous measurements of the signal at two antennas are required.
Another approach was to switch the receiver between two or more, typically three or four, antennas at an audio rate. The down converted signal was frequency modulation (FM) detected and the frequency shift, the rate of change of phase as a function of time, thus created was measured to determine the signal""s arrival angle. This technique is very useful with signals with a relatively coherent carrier, such as amplitude modulation or narrow band frequency modulation. This technique, however, does not yield good results when digital modulation techniques obscure the carrier phase. This is especially true when spread spectrum techniques are used. In that case care is taken to make the carrier appear noise-like.
See xe2x80x9cAntenna Engineering Handbookxe2x80x9d 2d. Ed., McGraw-Hill Book Company (1961) by Johnson and Jasik at pages 39-12 through 39-16, especially FIG. 39-10, for descriptions of the above-mentioned types of direction finders.
What is desired is a multi-channel direction finder system that accurately indication the direction to an interfering signal source where the signal is a digitally modulated suppressed carrier signal.
Accordingly the present invention provides a direction finder system using spread spectrum techniques that modulates each antenna channel with a PRN code unique in value or time. The encoded channels are summed and processed by a single demodulator and digitized. The resulting IF signal is analyzed by a digital signal processing engine using corresponding PRN code values or times to determine a phase or time of arrival of each channel relative to a reference channel. From the phases and/or times of arrival an accurate direction to the signal source is determined.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in light of the appended claims and attached drawing.